Methods and systems disclosed herein relate generally to predicting a vessel's trajectory, and more specifically, to predicting the trajectory of an underwater vehicle.
Daily global ocean forecasts that include a four-dimensional (4d) (latitude, longitude, depth, and time) estimation of ocean currents can be generated. An approach taken for the estimation of vehicle position over time is to start with a known position from infrequent fixes (Global Positioning System (GPS), Ultra-short Baseline (USBL), terrain-based, etc.) and use the vector sum of the vehicle velocity (heading and speed through the water) with the forecast current.
Validation of this approach can be accomplished using log data that was received from underwater gliders which provides GPS positions at each dive and surfacing point. An underwater glider propels itself using a buoyancy engine and wings that create lift to produce horizontal motion. From a vehicle motion modeling perspective, an underwater glider must have vertical motion to move horizontally. Since underwater gliders do not use engines for propulsion they generally have substantial endurance suitable for ocean sampling, underwater plume tracking, and sustained surveillance. However, these vessels are slow, with sustained horizontal speeds typically below 0.5 m/s, and navigating them is challenging as ocean currents can exceed 2 m/s.
The Naval Coastal Ocean Model (NCOM) was developed to generate daily global ocean forecasts predicting temperature, salinity and currents. FIGS. 1 and 2 show representative current forecasts during underwater glider deployment exercises. In these figures color 303 represents current speed in m/s and arrows 301 indicate the current direction. FIG. 1 shows the current at the surface with speeds as great as 0.8 m/s. FIG. 2 shows the current at 1000 m, the maximum depth of the glider dives, where the speed is predominately below 0.02 m/s.
Position estimation for underwater vehicles operating in the open ocean can be problematic with existing technologies. Use of GPS can require the vehicle to surface periodically which poses a potential navigation hazard and subjects the vehicle to the faster currents near the surface. Inertial systems can be ineffective without the use of Doppler Velocity Logs (DVL) whose ranges can be too limited for deep ocean operation unless the vehicle is very near the seafloor. Surface or bottom mounted transponder systems can be expensive to deploy and restrict the geographic area that the vehicle can operate in. A ship equipped with a USBL system can be used to track an underwater vehicle, which can be an expensive option for long deployments.
A complication in the open ocean is that position estimation is problematic while submerged. Glider depth can be directly measured by the vehicle using a pressure sensor. Vertical velocity can be derived from depth versus time, and horizontal speed through the water can be estimated given vertical velocity, vehicle pitch angle and a parameterized hydrodynamic model for the vehicle. Consequently, the only certain position information, for purpose of simulation, is depth (as a function of time), the time of the dive and the starting and ending surface positions. In the present embodiment, the motion model can use initial simplifying assumptions including zero hydrodynamic slip between the vehicle and ocean current and a symmetric V shaped flight trajectory. For the simulations conducted, the maximum depth of the dive and the time of the dive can be used to compute an estimate of a single vertical velocity. Beyond this model, sources of error in position prediction can include errors in the forecast currents, hydrodynamic slip and deviations of the vehicle from the commanded heading, horizontal and vertical speeds. Variations in the vehicle commanded motion can include factors such as putting the processor to sleep periodically to save power (so heading is not strictly maintained), variations in vertical speed due to changes in water density, and other than symmetric dive profiles.
What are needed are a system and method for estimating the vessel's position while it is underwater that improves on a simple straight line dead-reckoned estimate.